1. Field of the Invention
The present invention relates to a photocatalyst and to a process for producing the same.
2. Related Background Art
Methods employing photocatalysts are among the existing techniques for achieving effective utilization of solar energy. Photocatalysts are substances that function as catalysts activated by light irradiation, typical known ones including homogeneous catalysts which employ metal complexes and non-homogeneous catalysts which employ semiconductor catalysts.
The mechanism by which a photocatalyst exhibits a catalytic effect upon light irradiation is theorized to be as follows, for semiconductor catalysts, Specifically, a semiconductor has a band structure in which conduction bands and valence bands are partitioned by forbidden bands of appropriate width, and irradiation of light of energy exceeding the band gap causes excitation of the valence band electrons to the conduction bands. As a result, electron holes are produced in the valence bands and electrons in the conduction bands, and these electron holes and electrons induce an oxidation or reduction reaction by a mechanism similar to electrolysis.
The band gap size and the potential of the conduction band and valence band are important factors contributing to the catalytic activity. In addition to these, other contributing factors include the life and ease of movement of the generated electrons and electron holes, as well as the charge separation, and the overvoltage and reactivity site of the oxidation-reduction reaction.
As photocatalysts with such functions there are known semiconductor catalysts such as TiO2, ZnO, Ta2O5, CdS, GaP, SiC, K4Nb6O17, K2La2Ti3O10, Na2Ti6O13, BaTi4O9 and K3Ta3Si2O13, and it has been confirmed that hydrogen and (oxygen are produced when these catalysts are powdered, suspended in water and irradiated with light.
With certain types of photocatalysts, it has been confirmed that formic acid, formaldehyde, methanol, methane and the like are produced upon light irradiation while carbon dioxide is passed through their aqueous suspensions. It has also been reported that catalysts with high dispersion of the semiconductor catalyst TiO2 exhibit activity in pores of the insulator zeolite during the conversion reaction of carbon dioxide to hydrocarbons.
In addition to those mentioned above, titanosilicate zeolites (TS-1, TS-2) are also known as problem which hampers their practical implementation.
It is an object of the present invention, which has been accomplished in light of such technical problems, to provide a photocatalyst which can induce efficient photocatalytic reaction even when used in a small amount and with a small light irradiation area, and which can thereby decompose water and the like at an adequate reaction rate. It is another object of the invention to provide a process for producing the photocatalyst.
As a result of much diligent research aimed at achieving the aforementioned object, the present inventors have discovered that a photocatalytic function is exhibited by porous materials composed of compounds with a basic framework having titanium atoms and phosphorus atoms bonded by way of oxygen atoms and porous materials composed of compounds with a basic framework having zirconium atoms and phosphorus atoms bonded by way of oxygen atoms. The present invention has been completed upon the further discovery that such photocatalysts can be used for efficient photocatalytic reaction even in small amounts and with a small light irradiation area, thus allowing decomposition of water and the like at a sufficiently high reaction rate.
In other words, the photocatalyst of the invention is a photocatalyst comprising a porous material, materials exhibiting catalytic activity in photocatalytic reactions. Such zeolites are porous materials with pares (micropores) of generally 0.3-1.3 nm in size, and it is believed that their catalytic activity is exhibited by their moderate solid acidity. In recent years it has become possible to) synthesize titanosilicate mesoporous materials with larger pores mesopores) than zeolite, measuring 1.5-50 nm in size, and such substances have also been reported to exhibit activity in photocatalytic reactions.
Because these photocatalysts allow utilization of solar energy for sundry chemical reactions, they are being studied for effective use in numerous fields, such as production of hydrogen and oxygen by decomposition of water, synthesis of hydrocarbons such as methane and methanol from carbon dioxide and water, and purification of harmful substances such as NOx and dioxin.
Nevertheless, these photocatalysts of the prior art have slow reaction rates in photocatalytic reactions for decomposition of water or fixing of carbon dioxide, and since in order to adequately accelerate the reactions it has been necessary to use large amounts of catalyst and accomplish light irradiation over a wide area, this has constituted a wherein the porous material comprises a compound with a basic framework having metal atoms bonded to phosphorus atoms by way of oxygen atoms, the metal atoms being selected from the group consisting of titanium atoms and zirconium atoms.
The present invention includes the first to third preferred embodiments of the photocatalyst as described below.
First Embodiment
The first embodiment of the present invention is a photocatalyst comprising a porous material, wherein the porous material comprises a compound with a basic framework having titanium atoms bonded to phosphorus atoms by way of oxygen atoms and the porous material has a median pore diameter of from 0.2 nm to less than 1.5 nm.
It is believed that in the photocatalyst according to the first embodiment, which has a basic framework with titanium atoms and phosphorus atoms bonded by way of oxygen atoms, the electron holes and electrons produced by light irradiation efficiently contribute to the catalytic reaction so that the efficiency of the catalytic reaction is enhanced even with a small light irradiation area. Also, since the photocatalyst of the first embodiment has pores of the size specified above (micropores), it is possible to greatly increase the contact area for contact of water, for example, thereby increasing the number of reaction sites for the photocatalytic reaction and allowing efficient decomposition of water even with a small photocatalyst amount.
The basic framework of the photocatalyst of the first embodiment preferably has the composition represented by the following general formula (1).
TixPyOzxe2x80x83xe2x80x83(1)
wherein x is a number of from 0.5 to 1.5, y is a number of from 0.5 to 1.5 and z is a number of from 3 to 6.
A photocatalyst of the first embodiment having the composition represented by general formula (1) above exhibits improved efficiency of light absorption and efficiency of the light irradiation-induced charge separated electron holes and electrons participating in the photocatalytic reaction.
According to the first embodiment, the porous material preferably has a pore volume of from 0.05 to 0.5 ml/g. If the porous material has a pore volume within this range, the contact area will tend to increase for contact with water, for example, thereby increasing the number of reaction sites for the photocatalytic reaction and tending to increase the efficiency of the photocatalytic reaction.
Second Embodiment
The second embodiment of the present invention is a photocatalyst comprising a porous material wherein the porous material comprises a compound with a basic framework having titanium atoms bonded to phosphorus atoms by way of oxygen atoms and the porous material has the median pore diameter of from 1.5 nm to 30 nm, further wherein the value of the total volume of pores with a diameter in the range of xc2x140% of the median pore diameter divided by the total pore volume is from 0.4 to 1.
Since the photocatalyst according to the second embodiment is composed of a compound with a basic framework having titanium atoms and phosphorus atoms bonded by way of oxygen atoms, it is possible to incorporate numerous titanium atoms which participate in the catalytic reaction into the compound, to exhibit more excellent activity as a photocatalyst. The photocatalyst of the second embodiment also has a very high surface area due to the mesopores described above, and since this provides numerous reaction sites, high activity can be exhibited even with a low amount of photocatalyst.
The mesopores allow adsorption of substances of a size that cannot easy penetrate micropores, so that catalytic action can be exhibited for substances that do not easily undergo photocatalytic reaction with titanosilicate zeolites and the like. Furthermore, since the photocatalyst of the second embodiment has highly uniform pore diameters as explained above, it is possible to achieve a selective catalytic effect for substances of a specific size, while also shortening the time required for the catalytic reaction.
The photocatalyst of the second embodiment preferably has the composition represented by the following general formula (2).
TiPmOnxe2x80x83xe2x80x83(2)
wherein m is a number of from 0.1 to 1.5 and n is a number of from 2 to 5.
When the photocatalyst of the second embodiment has a composition represented by general formula (2), the basic framework having titanium atoms and phosphorus atoms banded by way of oxygen atoms efficiently acts for catalytic reaction so that the catalytic efficiency is improved.
A photocatalyst according to the second embodiment preferably has its basic framework modified with alkyl groups, where the alkyl groups are bonded to the phosphorus atoms. Modifying the basic framework with alkyl groups can render the porous material surface and pore interiors hydrophobic, to exhibit an excellent photocatalytic effect even for highly hydrophobic substances.
In the photocatalyst of the second embodiment, the pore walls serving as partitions between adjacent pores are preferably crystalline, and the pore walls serving as partitions between adjacent pores also preferably exhibit an X-ray diffraction pattern with at least 2 peaks at a diffraction angle of at least 10xc2x0. When the pore walls exhibit this characteristic, the alignment of the atoms of the porous material will tend to be regular, resulting in highly regularly arranged pores and improved activity for use as a catalyst.
In the aforementioned first and second embodiments, the porous material preferably has an ion-exchange capacity of from 0.01 to 10 mmol/g. In a photocatalyst according to the first or second embodiment, the bonded states of the constituent atoms (number of bonds and coordinated structure) can vary considerably when the titanium atoms and phosphorus atoms which have different valencies form the basic framework by way of oxygen atoms, and the basic framework thus becomes charged or polarized. Consequently, the photocatalyst of the first or second embodiment will exhibit a cationic exchange property and/or anionic exchange property. Therefore, the ion-exchange capacity can be adjusted to within the range specified above.
Third Embodiment
The third embodiment of the present invention is a photocatalyst comprising a porous material, wherein the porous material comprises a compound with a basic framework having zirconium atoms bonded to phosphorus atoms by way of oxygen atoms, further wherein the compound has no organic group-crosslinked structure and the porous material has the median pore diameter of from 0.3 nm to 2 nm.
Since the photocatalyst of the third embodiment is composed of a compound with a basic framework having zirconium atoms and phosphorus atoms bonded by way of oxygen atoms and having no organic group-crosslinked structure, as mentioned above, it is possible to incorporate numerous zirconium atoms which participate in the catalytic reaction into the porous material, to achieve sufficiently high catalytic activity when it is employed as a catalyst. The photocatalyst of the third embodiment also has an adequately high surface area since its pores have a median pore size within the above specified range, and since this provides numerous reaction sites, sufficiently high catalytic activity can be achieved even with a low amount of catalyst,
In the photocatalyst of the third embodiment, the value of the total volume of pores with a diameter in the range of xc2x140% of the median pore diameter divided by the total pore volume is preferably from 0.4 to 1. When the pore volume satisfies this condition, the pore sizes are highly uniform and the shape selectivity of reaction substrates for catalytic reaction will tend to be higher.
The basic framework preferably has a composition represented by the following general formula (3).
ZrPaObxe2x80x83xe2x80x83(3)
wherein a is a number of from 0.1 to 10 and b is a number of from 2 to 10.
When the photocatalyst of the third embodiment has a basic framework with a composition represented by general formula (3), the basic framework will efficiently function for improved catalytic efficiency in catalytic reactions.
The present invention also provides a process for producing photocatalysts according to the aforementioned second and third embodiments.
Specifically, a photocatalyst according to the second embodiment may be produced by a process including a step of reacting a titanium-containing compound and a phosphorus-containing compound in water in the presence of an alkylamine and hydrofluoric acid, or by a process including a step of reacting a titanium-containing compound and a phosphorus-containing compound in water in the presence of an anionic surfactant.
In the production process using an alkylamine, the molar ratio of the hydrofluoric acid with respect to the phosphorus-containing compound is preferably from 0.3 to 1. When the molar ratio of the phosphorus-containing compound and the hydrofluoric acid is a value within this range, the crystallinity of the resulting photocatalyst is improved, and therefore the regularity of the pore arrangement is increased and the photocatalytic activity will tend to be enhanced.
In the production process using an anionic surfactant, the reaction between the titanium-containing compound and the phosphorus-containing compound is preferably carried out at a pH of from 1 to 6. Conducting the reaction in this pH range will tend to increase the regularity of the pore arrangement of the photocatalyst, and enhance its catalytic activity.
In both of these production processes, the phosphorus-containing compound preferably contains an alkylphosphonic acid and/or an alkylphosphonic acid ester. When the phosphorus-containing compound contains an alkylphosphonic acid and/or an alkylphosphonic acid ester, the resulting photocatalyst will be modified with alkyl groups, and since this will improve the hydrophobicity on the porous material surface and in the pore interiors, a satisfactory catalytic effect will be exhibited even for highly hydrophobic substances.
Both of the aforementioned processes can introduce a P+xe2x80x94Xxe2x88x92 ion pair and/or a Pxe2x80x94OH bond onto the phosphorus atoms of the basic framework of the porous material. When a P+xe2x80x94Xxe2x88x92 ion pair is introduced, the Xxe2x88x92 ion allows ion exchange with other anions, thereby imparting an anionic exchange property to the porous material. When a Pxe2x80x94OH bond is introduced, the Pxe2x80x94OH bond polarizes into Pxe2x80x94Oxe2x88x92xe2x80x94H+, and the H+ ion allows ion exchange with other cations, thereby imparting a cationic exchange property to the porous material.
The process for producing a photocatalyst according to the third embodiment includes a step of reacting a zirconium-containing compound with a phosphorus-containing compound in water in the presence of a diaminoalkane and alcohol. According to this process, it is possible to efficiently and reliably obtain a photocatalyst according to the third embodiment.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope at applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.